Why Does Dog Poop Smell Bad To Us But Good To Dogs?

Why Does Dog Poop Smell Bad To Us But Good To Dogs?

There are some things we can all agree on. For instance: The fact that poo smells bad. There are some contrarians out there — fetishists, primary school class clowns and so on — but for the most part this issue transcends the usual divisions.

Illustration: Jim Cooke/Gizmodo

Dogs, on the other hand… Well, it’s hard to tell where they stand. Some of them like it. Some of them don’t like it. Some of them roll around in the stuff for fun. If you were to generalise, you could say that, as a species, dogs at least seem more interested in the stuff. They don’t just reflexively turn their noses at it. Often, they burrow their noses in it.

We don’t tend to think much about this state of affairs, which makes sense. But probe a little deeper, and a whole reeking world opens up — as we learned when, cringing a little, we asked a number of top psychologists, scent-scientists and dog-experts to talk about it. In the latest instalment of Giz Asks, we find out that our species’ disagreement vis-a-vis the appeal of sniffing street-turds gets at some crucial questions regarding evolution, cultural learning, and what it really means to “like” or “dislike” a smell.

Donald Wilson

Author of ‘Learning to Smell: Olfactory Perception from Neurobiology to Behaviour’, Johns Hopkins U. Press, Professor of Child & Adolescent Psychiatry and of Neuroscience & Physiology, Senior Research Scientist, Nathan Kline Institute for Psychiatric Research

Do dogs really enjoy sniffing faeces, and urine, and other dog’s butts? It looks like they might kind of like it, because the dog never goes up and just gets the aroma off another dog’s butt — very often there is contact there.

But a lot of animals have a second olfactory system that’s geared towards dealing with odours that are really heavy. The odours that we smell are by definition volatile — they float up into the air, and as we’re breathing they go up our nose. Dogs have a structure called the vomeronasal organ, and that works not by just inhaling, but it’s actually sort of like a little pump. It’s in the nose, or sometimes it’s in the roof of a mouth. They can bury their noses into another dog’s butt, or into urine, or poop, [and with] their vomeronasal can really suck up these heavy molecules and extract more info out of it than we can.

Peter Hepper

Head of the School of Psychology at Queen’s University, Belfast

For the dog, faeces are a source of a huge amount of information. They tell dogs about the individual who deposited the faeces, their dominance status, relatedness, sex and so on. One key function for wild canines is delineation of territory. Dogs defecate around territory boundaries and key paths to mark ownership with faeces. Dogs use faeces, humans use fences — although admittedly I am unaware of any experiments to test the effectiveness of human faeces as boundary markers. Faeces are an attractant to dogs and one of the great mysteries yet to be solved is why some of them roll in it. Perhaps to camouflage their own odours, enhancing their social standing. Or perhaps there is no function — just a remnant from evolution or, interesting in terms of the perceived value of faeces, a comforting/reinforcing function.

Evolution has probably contributed to this, the greater range of smells a dog can detect has co-evolved with the evolution of olfactory signals to provide a system of perception that places much more emphasis on odours. Human reliance on vision placed little pressure on us evolving a system of olfactory signals comparable to the dog due to the differences in the way we operate in the world.

Humans perceive “bad” odours through either some inbuilt evolutionary acquired mechanism to prevent harm, e.g. repulsion by faeces to prevent disease, or through learning. A particularly powerful source is 1 trial food aversion learning where if you become ill after consuming a particular food the smell taste of the food becomes aversive and remains so for a long time. In both cases the function appears to be to protect us from harm and have evolved to do so. Some chemicals smell bad — e.g. ammonia and this actually “feels” bad in the nose when experienced.

Alexandra Horowitz

Author of #1 NYT Besteller ‘Inside of a Dog’ and ‘Being a Dog’; teaches seminars in canine cognition and creative nonfiction writing at Barnard College, Columbia University, and heads the Horowitz Dog Cognition Lab.

So-called “bad” and “good” smells are products of our culture. As young children we are ambivalent toward smells like poo and stinky feet: We have to be taught that these are “bad”. Surely there is some biological predisposition to avoid smells which come from sources we should not ingest, but infants are surprisingly open-minded about so-called bad smells.

Dogs, by contrast, are in but not of our culture. They do not inherit our value system (unless we explicitly train them in its rules) and so are left with their own canine tendencies. And their tendencies are defined by their being olfactory creatures — animals who smell the world first, unlike humans. The smellscape of the footpath for an urban dog, for instance, draws a picture of who’s been by, when, what they have eaten, and where they have gone. For dogs, there seem not to be good nor bad (with a few exceptions) smells; smells are just the way the world looks. Smells are just information — poo smells included (although most dogs are averse to their own excreta).

Cat Warren

Cat Warren is the author of the New York Times bestseller What the Dog Knows: Scent, Science, and the Amazing Way Dogs Perceive the World.

You have this whole sort of marvellous range of behaviours — and even different behaviours with different dogs. It’s supposed to be one of the most disgusting things, when dogs eat their own poo, or eat another dog’s poo — but very often it’s because they’re actually missing some essential vitamins and minerals in the food they’re being given. The poo gives them that.

If we think about the human gut versus a dog gut, one of the reasons that dogs do a lot better smelling and eating stuff that would probably kill us is that their digestive system is so short that it moves right through them. Humans have learned that these kinds of things are bad news; dogs won’t get punished very often by their bodies for eating really smelly stuff.

Avery Gilbert

Fellow of the Association for Psychological Science; author of What the Nose Knows: The Science of Scent in Everyday Life, which was a finalist for the Los Angeles Times Book Prize in Science & Technology

What makes a “bad” smell? There’s always an Uncle Fester in the crowd who likes skunk. And context matters: A ripe French cheese is fine, but the same smell coming from a full diaper is not. We’ve evolved to dislike not specific smells but classes of smell linked to biologically relevant sources: Decaying meat, urine, and of course faeces.

Not all shit smells the same. That of predators has a dense, meaty character. The smell used to put our hominid ancestors on their guard which may be why it still resonates with us. Cow pies and horse manure are much less offensive, partly because of diet but also because we have co-evolved with these creatures for thousands of years.

Dogs don’t approach shit as an aesthetic experience — they treat it as a source of social information, like an olfactory Instagram. It answers a lot of questions: Who left it? How recently? Is the pooper healthy? We are able to extract similar information. The lingering cloud in the office restroom tells you who had lunch at P.F. Chang’s. Plumbing and ventilation rob us of the social signals in faeces and leave us with mere disgust.

Charles Spence

Head of Oxford University’s Crossmodal Research Laboratory. His new book ‘Gastrophysics: The new science of eating’ (2017; Penguin Viking) deals, in part, with the translation of the latest neuroscience-inspired design for food from the modernist restaurant to the healthcare/aeroplane/home setting, etc.

We’re all born liking certain tastes and disliking certain others. But we’re not really born with any preferences as far as smells go. The reason most smells, when we think about them as adults or grown children, would acquire that like or dislike is because of what those smells are paired with, or associated with. In young infancy babies will put all sorts of stuff in their mouth, and they have to learn over time that this is disgusting, this is disliked… Everything is learned.

There is the faecal note, though, as they call it in perfumery — a pleasant faecal note in the base of some perfumes. Mixed with flowery and floral stuff, the faecal note keeps you interested. We can adapt to pleasant smells, we can adapt to neutral smells, but we never really adapt to unpleasant smells. So those who live next door to the chicken factory will always smell horrible, for all of their lives. If you put a slightly unpleasant note in a perfume or fragrance, then you never quite adapt to that, and hence you’re kind of aware of the fragrance for longer than you’d otherwise be. If it was just pleasant you’d say, “OK, pleasant, yeah, I’ve got it, I don’t need to pay attention to that any more.”

Dr Deborah Wells

Reader, School of Psychology, Queen’s University, Belfast

As humans, we are biologically programmed to avoid ingesting materials that might be dangerous to our health. From an evolutionary perspective, it is simply not advantageous to eat something that is potentially harmful; indeed, in this respect, faeces, which are ridden with bacteria, could be considered to smell bad for our own good, reducing our chances of coming into contact with infection and disease.

In reality, faeces often smell aversive to many animals, including dogs. However, a small proportion of individuals may be intrigued by, or even attracted to, the smell of faeces, in some cases going so far as to consume them. The dog has a remarkable sense of smell, thought to lie anywhere between 10,000-100,000 times better than that of humans. Pet dogs are often provided with a very rich meat-based diet, rendering the end product appealing to some animals. Like its ancestor, the wolf, the domestic dog is an opportunistic scavenger and may navigate towards the smell of faeces, particularly those containing remnants of undigested food. Some dogs also urinate after defecation, with urine containing a source of wealthy information about the status of the individual, e.g., its sex, castration status. Again, this may serve as a lure for a social animal that relies heavily on its sense of smell.

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